Although a relatively recent addition to communication technology, time division multiple access (TDMA) has become an acceptable technique for use with transponders located in synchronous or quasi-synchronous earth orbit. Conventionally, a transponder has an associated up-link frequency on which it receives from a plurality of stations. The transponder further has a down-link frequency on which it transmits. Time division techniques place a number of channels in a single frequency. To allow multiple stations to talk to each other through a single transponder on common up-link and down-link frequencies, proper timing is essential. To prevent garbling, the transmissions from each of the different stations must be received in time sequence at the transponder. In order to efficiently utilize the capacity of the transponder, proper timing is achieved through timing from a common marker or reference burst transmitted to each of the stations through the transponder. The reference burst defines a frame which is further conceptually subdivided into a plurality of time slots. Each of the stations is allowed to transmit, in burst form, and each burst is aimed at a different slot. Thus, the frame includes the reference burst, and a plurality of slots for traffic. To make control of the network tractable, a further portion of the frame is provided for supervisory signalling purposes, i.e., to allow changing the slot allocations among the stations, etc.
One typical control problem that must be solved is contention between the stations for available traffic slots. The solution of the control problem must also take into account the round trip propagation time which is on the order of 300 msec. The round trip propagation time only becomes a complicating factor where slot allocation control is distributed. While it has been recognized that centralized slot allocation or real location is simpler than distributed control, centralized control presents significant disadvantages. As a result, distributed control is presently favored.
One manner in which the contention for traffic slots has been solved, in prior art single transponder systems, has been to provide each station, at all times, with information regarding which traffic slots are in use and which traffic slots are available. A station desiring to transmit in what it believes to be an available slot, first includes a message in a supervisory signalling time slot requesting the desired slot. That request is transmitted, through the transponder, to all other stations, including the transmitting station. Because of the round trip propagation delay the available information regarding traffic slots in use can be as much as 300 msec. "old". Therefore, after requesting a traffic slot, a station must delay using that slot until it hears its own transmission. If, in the time between it transmits its request, and the time it hears its request, it does not receive a request from another station for that traffic slot, then it can safely assume that it has received access for that traffic slot. Significantly, this procedure requires that each station "hear" transmissions from all other stations.
Present technology makes available transponders which have been optimized and have available a fixed bandwidth. This, of course, fixes the amount of traffic that can be funnelled through the transponder. For networks which have a greater need than a single transponder can supply, obviously, multiple transponder arrangements are required. Such arrangements necessitate multiple up-link and down-link frequencies. Each station in such a system may include separate apparatus, i.e., to transmit and to receive and to control the transmission and reception for each transponder. As disclosed in my prior U.S. Pat. No. 3,838,221, issued Sept. 24, 1974, multiple transmitters and receivers may be eliminated and each of the stations in a multi-transponder network may employ a plurality of up-link converters and/or a plurality of down-link converters, s well as a common TDMA control for each transponder, so that each of the stations can transmit to and receive from each of the transponders. However, as is further disclosed in the referenced patent, all stations need not have this equipment and certain of the stations can be restricted to communicating with certain of the transponders. For example, if the traffic through a particular station is expected to be relatively light, that station may be restricted to one or more transponder, less than all the transponders in the network by not providing that station with the necessary up- and down-link converters for the remaining transponders, and eliminating the corresponding TDMA control. At the same time, any station desiring communications with a particular station which is limited in the transponders available to it must, of course, communicate with that station through one of the available transponders. This is undesirable since it reduces system flexibility.
The technique of employing multiple transponders in a network, wherein a plurality of stations can transmit to or receive from more than a single transponder, is referred to as "transponder hopping" and is further discussed in my article "The Application of TDMA to the INTELSAT IV Satellite Series" appearing in the COMSAT Technical Review, Volume 3, Number 2, pages 257-75 (Fall 1973); see in particular pages 270-72.
The referenced patent also discloses that a single or common TDMA control can be utilized by synchronizing the different frames of the different transponders. However, the prior art does not teach a control procedure which establishes a signalling channel necessary for transponder hopping. For example, in up-link hopping where contention for available traffic slots is possible, a signalling channel is required to avoid simultaneous slot seizure. The known solution to simultaneous slot seizure requires each station to hear requests for traffic slots from other stations. Transponder hopping, however, implies that a station is transmitting into a transponder from which it may not receive, and thus cannot hear either its own transmission or, transmissions from other stations transmitting into that transponder. As a result, the prior art techniques for preventing simultaneous slot seizure cannot be applied.
On the other hand, in down-link hopping a signalling channel is necessary to alert a station to listen to a particular transponder at the appropriate time. In addition, of course, all stations must be informed of changes in slot timing and duration even if they may not be "listening" when the orders are transmitted.
Thus, a basic difficulty to be overcome is the integration of plural transponder networks having due regard for both optimum configuration and a solution to the control problem. For example, a station requires control apparatus, transmit and receive apparatus and an antenna to communicate in a TDMA mode with other stations in a network through a remote transponder. It is possible to duplicate one set of such apparatus at each station for each transponder. Obviously, this is not an optimum configuration and minimizing equipment duplication is desirable.
Before proceeding further, it is worthwhile to segregate TDMA communication systems into single channel per burst systems vs. multiple channel per burst systems since the specific requirements of these systems differ. In a single channel per burst (SCPB) a station, which may serve a plurality of users, acquires a specific slot in the TDMA frame in response to a specific user request. The link is established for the user and then the link is broken and the slot relinquished at the termination of the call. Multiple channel per burst systems (MCPB) on the other hand, merely allocate a portion of the station's burst to a user in response to a request, i.e., the link exists prior to the user's request.
Therefore, in SCPB, the network problem relates to the need for a station to acquire a slot and establish/disestablish a link, i.e., the called station must also acquire a slot for any return traffic. In MCPB, the problem relates to how variations in time slot duration (capacity) are handled. In both SCPB and MCPB there is, in addition, a need to alert the called station to the calling station's request for communication.
The present invention deals with the solution of the problet in the SCPB case. A companion application deals with the MCPB system.
It is therefore one object of the invention to provide control and signalling for multi-transponder communication networks which allow integration of distributed control apparatus for the plural transponders into one TDMA control. It is another object of the invention to provide such equipment in which equipment duplication is minimized by using transponder hopping techniques. It is another object of the invention to provide a method of operating multiple transponder TDMA networks which minimize the necessity for duplication of equipment.